The fight against pollution is often a piecemeal, nation-by-nation affair, which is problematic because pollution knows no national boundaries. But what's spreading isn't quite what everyone thought. Astudy published this month in Science tracked the movement of aerosal particles from around the world using the Calipso satellite, and found that half the aerosols in North America came from foreign sources.

The Calipso satellite allowed the researchers for the first time to track the aerosol movement in three dimensions, mapping specifically the interactions between Asia and North America. They were also able to identify the aerosols as dust, particles from combustion, and particles generated by ships on the ocean. After examining the data, they came to the conclusion that half of the aerosols in North America come from other nations — but interestingly, it's mostly dust, not pollution.

The researchers showed that while moving across the Pacific, the dust that was elevated 2-6 km above sea level survived the trip readily, around 56 teragrams annually arriving in our skies. Hitting this high, they're not influencing how breathable the air is. This dust is naturally generated, too, and isn't from human sources. Only 4 teragrams can be attributed to pollution, but this mass is much more effective at scattering and absorbing solar radiation than the dust is. Imported pollution may even be to blame for accelerating the the melting of snow in the Sierra Nevada.

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This casts the way we view fighting climate change on an international scale very differently — there's a lot more to do than just sequester carbon. As the study authors put it:

To mitigate aerosol impacts on regional climate change, actions by a single nation are inadequate. The world must work cooperatively and act synchronically to meet the challenges of global health on a changing planet. Focusing on the carbon budget and urban/industrial pollution sources is also inadequate because the imported dust dominates the mass budget and aerosol [direct radiative effects]. Dust emissions can respond to climate changes, such as changes of wind, precipitation, and vegetation. It is thus essential to acquire better understanding of the interactions between dust and climate.